Piezoelectric components may, for example in the form of piezo actuators, be formed as multilayer components with a number of respectively alternately arranged piezoelectric ceramic layers and electrode layers, and are gaining ever more importance in modern electrical engineering. For example, piezo actuators are used as adjustment drives in connection with valves and the like. Piezo actuators are also used for injecting fuel into a combustion space. Here, the requirements are increasing according to the increased requirements in terms of emission and consumption. Higher pressures and temperatures, as well as multiple injections, are requiring a higher thermal stability of the actuator as well as a high durability.
A known piezo actuator is described, for example, in DE 196 46 676 C1. In the case of the piezo ceramics used, use is made of the effect that they become charged under mechanical pressure, or tension, and on the other hand they expand when an electrical voltage is applied along the principal axis of the ceramic layer. In order to multiply the usable length expansion, for example, monolithic multilayer actuators are used, which comprise or consist of a sintered stack of thin films of piezo ceramic (for example lead zirconate titanate) with incorporated metal electrode layers. The electrode layers are alternately fed out of the stack and are electrically connected in parallel by means of external metallizations. To this end, a continuous strip-shaped or band-shaped external metallization, which is connected to all the electrode layers with the same polarity, is respectively applied on the two contact sides of the stack.
If an electrical voltage is applied to the external contact, the piezo films expand in the field direction. Owing to the mechanical series connection of the individual piezo films, the rated expansion of the entire stack is already achieved with relatively low electrical voltages.
In order to achieve an optimal excursion of the multilayer piezo-ceramic actuator together with minimal space requirement, it is favorable for the electrode layers to comprise the entire cross section of the actuator. This type of actuator is also known by the term fully active piezo stack. This means that electrode layers of alternating polarity are exposed on the lateral surfaces of the multilayer ceramic.
For this reason, it is necessary to provide these exposed electrode layers with suitable passivation. Passivation generally involves a nonmetallic protective layer, insulation layer or the like, by means of which electrical arcing and short circuits between neighboring electrode layers can be avoided. Without passivation, such arcing and short circuits could be caused, for example, by mechanical damage to the surface or contamination with impurities (for example grinding dust, fingerprints, and the like), moisture or fuels (for example diesel, rape methyl ester, and the like). Such damage and/or contamination may occur particularly during operation of the actuator, but also during the production process.
One possibility for this insulating layer is to apply a coating layer, which may comprise polyimide as material. The family of polyimides have been found most suitable for this use owing to their properties in respect of glass transition temperature, thermal stability and insulation properties. In the case of passivation with polyimide coating, however, during the application and curing process, layer thickness inhomogeneities and sometimes insufficient coverage at the edges (edge thinning) may occur. As a consequence, electrical voltage arcing can occur during operation.
One conventional approach is to carry out passivation of a piezo stack on all sides, each of the four sides being coated and cured separately. Piezo stacks on which there are poorly covered edges are subsequently rejected.
The passivation of piezo stacks on all four sides is very elaborate. This outlay is additionally increased by the material waste due to defective piezo stacks.
From WO 2002/089225, it is known to provide a passivation material for a piezoelectric component in multilayer design, which has a tear strength that is less than the adhesion strength of the passivation material on the electrical component. In this case, inter alia, plastics, polyurethanes, resins or polyimides are provided. From US 2007 024 7025 A1, it is also known to use glass as a passivation material.